1. Field of the Invention
The present invention relates to electronic circuits for driving inductive loads utilizing semiconductor power devices to drive the load and, more particularly circuits of this type monolithically integrated.
2. Description of the Prior Art
It is known that the driving of highly reactive loads presents transitory related problems. For instance, when a current through a grounded inductive load is suddenly interrupted, a negative overvoltage is generated across the load, caused by the discharge of the energy stored by the inductance during a charging phase (at the start of the current flow through the inductive load). Such a peak of negative voltage, if unchecked in terms of intensity, may cause the breakdown of junctions and, in case of monolithically integrated circuits, may cause the triggering of other transistors and of related problems. In this situation, the necessary intervention is that of providing means for discharging the energy stored in the inductance while limiting the negative overvoltage to an absolute value lower or equal to one VBE (Base-Emitter voltage).
According to known practices, there are substantially two types of circuits used for this purpose.
A first solution contemplates connecting a recirculation diode across the inductive load to provide a recirculation path to the current for discharging the stored energy.
Another solution contemplates recirculating the discharge current of the energy stored in the inductive load through the same power switching transistor, by connecting a control Zener diode between the base and the collector of the power transistor;
The first solution has the disadvantage of requiring at least an additional power device, i.e. the recirculation diode, having the capacity to withstand the passage of the peak discharge current.
The second solution, although using advantageously the same driving power transistor for recirculating the discharge current, requires the control Zener diode to be designed case by case in function of the supply voltage of the circuit in order not to interfere during the charging phase of the inductance.